vWF is produced in vascular endothelial cells or megakaryocytes, and is a blood coagulation factor in which a single subunit comprising 2,050 amino acid residues (monomers of about 250 kDa) are bound by an S—S bond to form a multimer structure (with a molecular weight of 500 to 20,000 kDa). The level thereof in the blood is about 10 μg/ml, and a high-molecular-weight factor generally has higher specific activity.
vWF has two major functions as a hemostatic factor. One of the functions is as a carrier protein wherein vWF binds to the blood coagulation factor VIII to stabilize it. Another function is to form platelet plug by adhering and agglomerating platelets on the vascular endothelial subcellular tissue of a damaged vascular wall.
Thrombotic thrombocytopenic purpura is a disease that causes platelet plug formation in somatic arterioles and blood capillaries throughout the whole body. In spite of recent advances in medical technology, the morbidity associated with this disease approximately tripled from 1971 to 1991. Pathologically, TTP is considered to result from vascular endothelial cytotoxicity or vascular platelet aggregation. Immunohistologically, a large amount of vWFs are recognized in the resulting platelet plugs, and vWF is considered to play a major role in causing them. A normal or high-molecular-weight vWF multimer structure is dominant in a TTP patient, and an unusually large vWF multimer (ULvWFM) or large vWF multimer (LvWFM) is deduced to play a major role in accelerating platelet aggregation or microthrombus formation under high shearing stress. In contrast, vWF was known to degrade at a position between residues Tyr 842 and Met 843 by the action of vWF-cleaving protease in the circulating blood of a healthy person under high shearing stress. Accordingly, TTP is considered to occur in the following manner. The protease activity in the plasma is lowered for some reason, and ULvWFM to LvWFM are increased to accelerate platelet aggregation. This forms platelet plugs in blood vessels.
Recently, Furlan et al. (Blood, vol. 87, 4223–4234: 1996, JP Patent Publication (Kohyo) No. 2000-508918) and Tsai et al. (Blood, vol. 87, 4235–4244: 1996) developed a method for assaying vWF-specific cleaving protease. In their report, this protease activity was actually lowered in TTP. The aforementioned authors reported that this enzyme was metalloprotease in the plasma and partially purified. However, they have not yet succeeded in the amino acid sequencing which would specify the protease. There have been no further developments since then.